Fluxes effective in suppressing non-wet-open at BGA assembly

ABSTRACT

The disclosure describes techniques for eliminating or reducing non-wet open (NWO) defect formation by using a low activity flux to prevent a solder paste from sticking to ball grid array (BGA) solder balls during reflow soldering. The low activity flux may be configured such that: i) it creates a barrier that prevents the solder paste from sticking to the solder balls of the BGA; and ii) it does not impede the formation of solder joints during reflow. In implementations, a solid coating of the low activity flux may be formed over balls of the BGA, and the BGA may then be bonded to a PCB during reflow. In implementations, the balls of a BGA may be dipped in a low-activity creamy or liquid flux prior to reflow. In some implementations, the flux may applied on a solder paste printed on pads of the PCB, followed by placement of a BGA.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.201810553189.5, filed on May 31, 2018.

TECHNICAL FIELD

The present disclosure relates generally to ball grid arrays.

DESCRIPTION OF THE RELATED ART

A ball grid array (BGA) is a surface-mount package that may be used topermanently mount and electrically connect components on a surface ofprinted circuit boards (PCBs). In a BGA, balls (sometimes referred to as“bumps”) of solder may be placed in a grid pattern on a substrate on anunderside of the package. The PCB onto which the BGA is mounted may havea matching set of metal pads (e.g., copper pads) for receiving thesolder balls to provide electrical interconnection.

With the continuing miniaturization of electronic components, the die ofthe BGA is getting thinner and the solder balls are getting smaller.Additionally, environmentally friendly manufacturing is driving the useof Pb-free solders, which increases the temperatures at which the BGA isprocessed to melt the solder balls during a bonding process. Due to acoefficient of thermal expansion (CTE) mismatch between the die andmolding compound of the BGA, this is increasing thermal warpage duringreflow processing.

These trends in the industry are increasing the challenges of boardassembly and introducing additional defects into the reflow solderprocess. One defect that has increased in incidence (e.g., due to a CTEmismatch) is the formation of a non-wet-open (NWO) defect at the BGA. ANWO defect, sometimes referred to as “non wet,” “lifted ball,” “hangingball,” “ball on pad,” or “ball on land,” generally refers a solder jointdefect whereby a BGA solder ball does not solder to a PCB pad (sometimesreferred to as a land pad).

FIG. 1 illustrates a surface-mount technology (SMT) reflow solderingprofile whereby a NWO defect develops. As shown, at stage 110, a solderball 101 of a BGA is placed on a solder paste 102 printed over a PCB pad111. Following stage 110, the assembly is heated. As the temperature ofthe assembly is heated, the shape of the BGA may begin to warp (e.g.,due to a mismatch of the CTEs between the die and molding compound ofthe BGA). Additionally, the shape of the PCB may be begin to warp. Asillustrated by stage 120, this thermal warpage may cause the solderpaste 102 to stretch and lift up from PCB pad 111, creating a gap 112and leaving the PCB pad 111 with little to no solder paste 102. Forexample, as the corners and edges of the BGA warp away from the surfaceof the PCB, solder paste deposits may adhere to the solder ball and belifted off the pad. This lifting may continue at a temperature range ofabout 160 to 190° C., below the melting temperature of the solder paste102. This lifting of paste 102 may subsequently cause a NWO defect.

At stage 130, during reflow, the lifted paste may melt and coalesce withthe melted solder ball to create mass 115 which is in the form of alarger solder ball. Additionally the exposed PCB pad 111 may exposed toa high reflow temperature, causing it to grow oxides that impedewetting. If the solder paste completely coalesces with the solder ballbefore the package collapses back down, it may not wet to PCB pad 111.When the assembly cools down, the BGA collapses, and the larger solderball may simply rest on PCB pad 111 without a physical connection,causing a NWO solder joint defect that has no electrical continuity(stage 140).

BRIEF SUMMARY OF EMBODIMENTS

Systems and methods are described for eliminating or reducing non-wetopen (NWO) defect formation by using a low activity flux to prevent asolder paste from sticking to ball grid array (BGA) solder balls duringreflow soldering.

In one embodiment, a method includes: dispensing solder paste on a padof a printed circuit board (PCB); mounting a ball grid array (BGA) onthe PCB to form an assembly, where mounting the BGA on the PCB includesmounting a flux coated solder ball of the BGA on the dispensed solderpaste, where the flux is a low activity flux; and reflow soldering theassembly to form a solder joint, where during reflow, the low activityflux coating the solder ball prevents the formation of a non-wet openbetween the solder joint and the pad, where the solder paste reflowsinto the solder ball to form the solder joint. During reflow, a flux ofthe solder paste dispensed on the PCB may penetrate through the lowactivity flux to remove oxides and promote wetting on the solder ball toform the solder joint.

In implementations, the method further includes: coating the solder ballof the BGA in the low activity flux to form the flux coated solder ballprior to mounting the BGA on the PCB. In implementations, coating thesolder ball of the BGA in the low activity flux includes: dipping thesolder ball in a bath of the low activity flux; spraying the lowactivity flux on the solder ball; or dispensing the low activity flux onthe solder ball.

In implementations, coating the solder ball of the BGA in the lowactivity flux further includes: curing the low activity flux to form asolid coating of the flux over the solder ball.

In implementations, coating the solder ball of the BGA in the lowactivity flux includes: placing the BGA on a tray, the tray including anopening at a bottom surface exposing the solder ball; and dipping theexposed solder ball of the BGA placed on the tray in a liquid or creamof the low activity flux.

In implementations, curing the low activity flux to form the solidcoating of the flux comprises baking the BGA in an oven at a temperaturebetween 140° C. and 160° C.

In implementations, the low activity flux is a no-clean flux and thesolder paste is a no-clean solder paste. In implementations where thelow activity flux is a no-clean flux, it may include: 15 to 85 wt % ofsolvents; 10 to 90 wt % resin; and 0.1 to 10 wt % of a thickening agent.In particular implementations, the low activity flux consistsessentially of: 30 to 40 wt % of butyl carbitol; 58 to 68 wt % of resin;and 1 to 3 wt % of a thickening agent.

In implementations, the low activity flux is a water soluble flux, andthe solder paste is a water soluble solder paste. In implementationswhere the low activity flux is a water soluble flux, the low activityflux may include: 15 to 85 wt % of solvents; 10 to 70 wt % oftetrakis(2-hydroxypropyl)ethylenediamine; 0.1 to 30 wt % of modifiedpolyethylene glycol ether; 0.1 to 20% wt % of a water soluble polymer;and 0.1 to 10 wt % of a thickening agent. In particular implementations,the low activity flux consists essentially of: 20 to 30 wt % of butylcarbitol; 30 to 40 wt % of tetrakis(2-hydroxypropyl)ethylenediamine; 20to 30 wt % of modified polyethylene glycol ether; 6 to 12 wt % of awater soluble polymer; and 3 to 6 wt % of a thickening agent.

In one embodiment, a method includes: dispensing solder paste on a padof a printed circuit board (PCB); dispensing a low activity flux on thesolder paste; mounting a ball grid array (BGA) on the PCB to form anassembly, where mounting the BGA on the PCB includes mounting a solderball of the BGA on the low activity flux dispensed over the solderpaste; and reflow soldering the assembly to form a solder joint, whereduring reflow, the low activity flux dispensed over the solder pasteprevents the formation of a non-wet open between the solder joint andthe pad, where the solder paste reflows into the solder ball to form thesolder joint. During reflow, a flux of the solder paste dispensed on thePCB penetrates through the low activity flux to remove oxides andpromote wetting on the solder ball to form the solder joint.

In one embodiment, a ball grid array (BGA) includes: a die; and asubstrate including an array of solder balls electrically coupled to thedie, where each of the solder balls of the array is coated in a solidcoating of a low activity flux. During reflow soldering of the BGA to aPCB, the low activity flux coating each of the solder balls may preventthe formation of non-wet open of a solder joint formed by reflowsoldering the solder ball to a solder paste dispensed on a pad of thePCB.

Other features and aspects of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, the featuresin accordance with embodiments of the invention. The summary is notintended to limit the scope of the invention, which is defined solely bythe claims attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The technology disclosed herein, in accordance with one or more variousembodiments, is described in detail with reference to the includedfigures. The figures are provided for purposes of illustration only andmerely depict example implementations. Furthermore, it should be notedthat for clarity and ease of illustration, the elements in the figureshave not necessarily been drawn to scale.

FIG. 1 illustrates a SMT reflow soldering profile whereby a NWO defectdevelops.

FIG. 2 illustrates an example BGA with which the technology disclosedherein may be implemented.

FIG. 3A is an operational flow diagram illustrating an example method offorming a solid coating of flux over solder balls of a BGA, inaccordance with implementations.

FIG. 3B, which illustrates a BGA, including solder balls, at variousstages of the method of FIG. 3A.

FIG. 4A is an operational flow diagram illustrating an example method ofbonding a BGA with low activity flux coated solder balls to a PCB, inaccordance with implementations.

FIG. 4B illustrates a solder ball of an example BGA and a pad of anexample PCB at various stages of the method of FIG. 4A.

FIG. 5A is an operational flow diagram illustrating an example method ofbonding a BGA to a PCB by dispensing a low activity flux on a solderpaste dispensed on the PCB pads, in accordance with implementations.

FIG. 5B illustrates a solder ball of an example BGA and a pad of anexample PCB at various stages of the method of FIG. 5A.

FIG. 6 is a photograph showing a BGA test board including a 9×9 array ofsolder balls.

FIG. 7A is a micrograph showing a normal solder joint formed between aBGA and PCB without an NWO defect at a PCB land.

FIG. 7B is a micrograph showing a solder joint formed between a BGA andPCB with a NWO defect, including a gap at an interface between the PCBland and joint.

FIG. 7C is a micrograph showing a solder joint formed between a BGA andPCB without an NWO defect at PCB land, using a low activity flux tosuppress NWO defect formation.

FIG. 8A illustrates a BGA solder ball without a flux coating.

FIG. 8B illustrates a BGA solder ball after dipping it in a low activitycreamy flux, in accordance with implementations.

FIG. 8C illustrates a BGA solder ball after forming a solid coating of alow activity flux over it, in accordance with implementations.

The figures are not intended to be exhaustive or to limit the inventionto the precise form disclosed. It should be understood that theinvention can be practiced with modification and alteration, and thatthe disclosed technology be limited only by the claims and theequivalents thereof.

DETAILED DESCRIPTION OF THE EMBODIMENTS

NWO defects have been ailing the electronics and semiconductorindustries for many years. As summarized above, the main cause of NWO isthe lifting of the solder paste from the PCB pads during reflow. Thisstage may occur when thermal warpage of the BGA is still fairly low.Some attempts have been made to develop specialty solder pastes toreduce the NWO defect. The success of these attempts has been limited.Although solder paste formulations can play an important role inregulating NWO defect formation, these pastes by themselves cannotovercome the NWO defect when thermal warpage is large. Other attempts toaddress NWO defects, including printing more solder paste on PCB pads(e.g., overprinting) and/or changing the reflow profile, have providedlittle success. For example, the industry has investigated lowtemperature soldering as an option to reduce thermal warpage that causesNWO defects. However, using lower melting temperature solders to formjoints may compromise solder joint reliability.

To this end, embodiments described herein are directed to eliminating orreducing NWO defect formation by using a low activity flux to prevent asolder paste from sticking to BGA solder balls during reflow soldering.The low activity flux may be configured such that: i) it creates abarrier that prevents the solder paste from sticking to the solder ballsof the BGA; and ii) it does not impede the formation of solder jointsduring reflow. In some implementations, a solid coating of the lowactivity flux may be formed over balls of the BGA, and the BGA may thenbe bonded to a PCB during reflow. In some implementations, the balls ofa BGA may be dipped in a low-activity creamy or liquid flux prior toreflow. In some implementations, the flux may applied on the printedsolder paste, followed by placement of a BGA. These and otherimplementations are further described below.

FIG. 2 illustrates an example BGA 200 with which the technologydisclosed herein may be implemented. In this example, BGA 200 isillustrated and described as a plastic ball grid array (PBGA), but itshould be appreciated that embodiments may be implemented using aceramic ball grid array (CBGA), a tape ball grid array (TBGA), a ceramiccolumn grid array (CCGA), or other suitable BGA type. As illustrated,BGA 200 includes a molding compound 210, a die 220, wire bonds 230, asubstrate 225, and solder balls 240. Die 220 is wire bonded to a topsurface of substrate 225 by wire bonds 230 and overmolded with a moldingcompound 210 (e.g., a plastic such as an epoxy based plastic). Attachedto a bottom side of substrate 225 is an array of solder balls 240. Eachof the solder balls in this example is coupled to a respective pad 235that may couple to an interconnect (not shown) that electrically couplesto die 200. In this manner, electrical signals may be conducted betweenthe die 220 and a PCB onto which BGA 200 is placed.

In example BGA 200, a solid coating of a low activity flux 250 has beenformed over each solder ball 240. During reflow soldering of BGA 200 toa PCB, because of its low activity, flux 250 may act as a barrier thatprevents a solder paste deposited on a pad of the PCB from sticking toand lifting up with solder ball. As a result, the solder paste may stayon a PCB pad (and subsequently reflow into the solder ball), therebypreventing an NWO defect from forming. Additionally, the coating of flux250 may be formulated such that it does not impede the formation of asolder joint during reflow. At reflow, a flux of a solder paste on thePCB pad may penetrate through the barrier of flux 250 and remove oxidesand promote wetting on solder ball 240 to form a solder joint.

In implementations, low activity flux 250 may be a “no-clean” flux whoseflux residue after reflow, may not need to be cleaned from the PCB toensure reliability. In such implementations, flux 250 may comprise 15 to85 wt % solvents (e.g., butyl carbitol), 10 to 90 wt % resin, and 0.1 to10 wt % of a thickening agent. In particular implementations as ano-clean flux, flux 250 may comprise 30 to 40 wt % of butyl carbitol, 58to 68 wt % of resin, and 1 to 3 wt % of thickener.

In implementations, low activity flux 250 may be a water soluble flux.In such implementations, flux 250 may comprise 15 to 85 wt % of solvents(e.g., butyl carbitol), 10 to 70 wt %tetrakis(2-hydroxypropyl)ethylenediamine (QUADROL) or derivativesthereof, 0.1 to 30 wt % of modified polyethylene glycol ether (LUTRONHF3), 0.1 to 20 wt % of water soluble polymers (e.g.,polyvinylpyrrolidone (PVP), Polyethylene glycol (PEG) 1000, etc.), and0.1 to 10 wt % of a thickening agent. In particular implementations as awater soluble flux, flux 250 may comprise 20 to 30 wt % of butylcarbitol, 30 to 40 wt % of QUADROL, 20 to 30 wt % of modifiedpolyethylene glycol ether, 3 to 6 wt % of PVP, 3 to 6 wt % of PEG 1000,and 3 to 6 wt % of thickener.

FIG. 3A is an operational flow diagram illustrating an example method300 of forming a solid coating of flux over solder balls of a BGA, inaccordance with implementations. Method 300 will be described withreference to FIG. 3B, which illustrates a BGA 350, including solderballs 355, at various stages of method 300. At operation 310, the BGAmay be placed on a tray 365 including an opening at the bottom forexposing solder balls 355. At operation 320, the exposed solder balls ofthe BGA placed on the tray may be dipped in a liquid or cream 370 of thelow activity flux to coat the solder balls in the flux. After coatingthe surface of solder balls 355 with a cream or liquid 370 of the flux,at operation 330 the flux coating may be cured to solidify the flux toform a solid flux coating 375. For example, depending on the compositionof the flux, the BGA 350 may be placed in an oven (e.g., a reflow oven)at a temperature of about 140 C to 160 C for 4 to 6 minutes. The oventemperature may be chosen to be substantially below the solidustemperature of the balls 355 of the BGA 350.

Although method 300 has been described with reference to forming a solidcoating of flux over solder balls of a BGA using a dipping process, itshould be noted that other techniques may be used to coat the solderballs in a low activity flux prior to curing. For example, in someimplementations the low activity flux may be jetted or otherwisedispensed on the solder balls of the BGA prior to curing.

FIG. 4A is an operational flow diagram illustrating an example method400 of bonding a BGA with low activity flux coated solder balls to aPCB, in accordance with implementations. Method 400 will be describedwith reference to FIG. 4B, which illustrates a solder ball 475 of anexample BGA 480 and a pad 485 of an example PCB 490 at various stages ofmethod 400. At operation 410, solder paste may be dispensed on pads 485of the PCB. For example, a solder paste 495 may be printed on each pad485 of the PCB 490. The printed solder paste may be a no-clean solderpaste or a water soluble solder paste. In some implementations, the PCB490 may be a component of a second BGA that is attached to BGA 480(e.g., as part of a package on package (POP) method). In someimplementations, the solder paste 495 may be an SnAgCu (“SAC”) solderpaste including an SAC solder (e.g., SAC305 or SAC387) and flux. In someimplementations, solder balls 475 may be SAC solder balls.

In implementations where a solid coating of flux has not been formedover solder balls of the BGA prior to assembly (e.g., as described abovewith reference to method 300), at optional operation 420, the solderballs of the BGA may be coated in low activity flux as described herein.For example, the solder balls 475 of the BGA 480 may be dipped in alow-activity creamy or liquid flux 476. In such implementations, theflux that is coated on the solder balls of the BGA may not be curedprior to placement of the BGA on the PCB (e.g., prior to operation 430).

In implementations where the solder paste deposited on the pads of thePCB is a no-clean solder paste, the flux coating the solder ball may bea no-clean flux. In implementations where the solder paste deposited onthe pads of the PCB is a water soluble solder paste, the flux coatingthe solder ball may be a water soluble flux.

At operation 430, the flux coated solder balls 475 of the BGA 480 mayeach be placed on a dispensed solder paste 495 of a respective land pad485 of the PCB 490 to form an assembly. At operation 440, the assemblymay be reflow soldered to form solder joints 499. During reflow, theflux 476 coating the solder balls 475 may be configured to act as abarrier that prevents the solder paste 495 deposited on a pad 485 of thePCB 490 from sticking to and lifting up with the solder ball 475 (e.g.,during thermal warpage of the BGA). As a result, the solder paste 495may stay on a PCB pad (e.g. pad 485) and subsequently reflow into thesolder ball 475 when the liquid solder ball collapses back onto theliquid solder paste (e.g., during cooling, when thermal warpagelessens). During reflow, a flux of a solder paste on the PCB pad maypenetrate through the barrier of low activity flux 476 and remove oxidesand promote wetting on solder ball 475 to form a solder joint 499.

FIG. 5A is an operational flow diagram illustrating an example method500 of bonding a BGA to a PCB by dispensing a low activity flux on asolder paste dispensed on the PCB pads, in accordance withimplementations. Method 500 will be described with reference to FIG. 5B,which illustrates a solder ball 575 of an example BGA 580 and a pad 585of an example PCB 590 at various stages of method 500.

At operation 510, solder paste 595 may be dispensed on pads of a PCB590. For example, a solder paste 595 may be printed on each pad 585 ofthe PCB 590. The printed solder paste may be a no-clean solder paste ora water soluble solder paste. In some implementations, the PCB 590 maybe a component of a second BGA that is attached to BGA 580 (e.g., aspart of a package on package (POP) method).

At operation 520, a low activity flux 576 may dispensed on the solderpaste 595 to form a flux coated solder paste. For example, the lowactivity flux 576 may be sprayed onto the solder paste 595. Inimplementations where the solder paste 595 deposited on the pads 585 ofthe PCB 590 is a no-clean solder paste, the flux 576 coating the solderpaste 595 may be a no-clean flux. In implementations where the solderpaste 595 deposited on the pads 585 of the PCB 590 is a water solublesolder paste, the flux 576 coating the solder paste 595 may be a watersoluble flux.

At operation 530, solder balls 575 of a BGA 580 (in this example,without a flux coating) may each be placed on the flux-coated solderpaste on a respective land pad 585 of the PCB 590 to form an assembly.At operation 540, the assembly may be reflow soldered to form solderjoints 599. During reflow, the flux 576 coating the solder paste 595 maybe configured to act as a barrier that prevents the solder paste 595deposited on a pad 585 of the PCB 590 from sticking to and lifting upwith the solder ball 575 (e.g., during thermal warpage of the BGA). As aresult, the solder paste 595 may stay on a PCB pad (e.g. pad 585) andsubsequently reflow into the solder ball 575 when the solder ballcollapses back onto the solder paste. During reflow, a flux of a solderpaste on the PCB pad may penetrate through the barrier of low activityflux 576 and remove oxides and promote wetting on solder ball 575 toform a solder joint 599.

Experimental Results

FIG. 6 is a photograph showing a BGA test board including a 9×9 array ofsolder balls. Experiments were conducted to check for NWO defects usingthis BGA test board including the 9×9 array of solder balls coated witha low activity flux in accordance with implementations. Tests wereconducted with three versions of the BGA test board. One BGA test boardwas dipped in the low-activity flux and then cured to form a solidcoating of the low-activity flux over the solder balls (e.g., asdescribed above with reference to FIG. 3). Another BGA test board wasdipped in the low-activity flux but not cured prior to testing. In bothcases, the low activity flux that was used was a no-clean flux having 35wt % butyl carbitol, 63 wt % resin, and 2 wt % thickener. In both cases,the solder balls of the BGA test boards were dipped in a 0.25 mm thickflux coating (about 50% of the height of the solder balls). The thirdversion of the BGA test board had no flux coating on the solder balls.

During testing, two different halogen-free no-clean type solder pastes,including SAC305 solder powders, were considered. The tested solderpastes were printed on the pads of each tested PCB. Each BGA test board(i.e., (i) solder balls dipped in flux, (ii) solid coating of fluxformed over solder balls, or (iii) flux not applied to solder balls) wasplaced onto the PCB pads with the printed paste to form a BGA/PCBassembly, and the BGA/PCB assembly was baked in a standalone baking ovenfor 8 minutes at 180° C., below the melting temperature of the solderpaste and solder balls. The BGA/PCB assembly was then cooled to roomtemperature. After cooling to room temperature, the BGA test board waspulled off the PCB, and the PCB land pads were inspected under amicroscope for NWO symptoms. PCB land pads that retained at least 70% ofthe solder paste on the pad were considered to be good at suppressingNWO defects.

Table 1, below, shows the results of testing.

TABLE 1 NWO Screening Test results, using BGA with 81 balls Average padsAverage pad % No. of pads with NWO per BGA Printed BGA Ball (81 with<70% symptom showing NWO paste balls per BGA) Bake time paste on pad perBGA symptom Paste A NO dipping or 8 mins 49, 81, 79, 38 61.8 76.2 solidcoating (180 ± 5 C.) Dipping with 5, 11, 11, 14 10.3 12.7 low-activityflux Solid coating of 6, 9, 14, 1 7.5 9.3 low-activity flux Paste B NOdipping or 8 mins 8, 9, 18, 1 9 11.1% solid coating (180 ± 5 C.) Dippingwith 0, 0, 0, 0, 0 0 0 low-activity flux Solid coating of 0, 0, 0, 0, 00 0 low-activity flux

As shown, on average, the PCB pads coated with solder paste A exhibiteda very high number of NWO defects (76.2% of the pads, on average, forthe four tested PCBs) when no low activity flux barrier was utilized(e.g., no dipping of the BGA solder balls in the low-activity flux). Bycontrast, the number of NWO defects, on average, were greatly reducedwhen using BGA test boards having solder balls dipped in thelow-activity flux (12.7% of the pads, on average) or solder balls dippedin the low-activity flux that was cured to form a solid coating (9.3% ofthe pads, on average).

On average, the PCB pads coated with solder paste B exhibited a lowernumber of NWO defects (11.1% of the pads, on average, for the fourtested PCBs) as compared with paste A when no low activity flux barrierwas utilized. When the solder balls of the BGA test boards were dippedin the low-activity flux, NWO defects were entirely eliminated whentesting with solder paste B.

FIG. 7A is a micrograph showing a normal solder joint 710 formed betweena BGA and PCB without an NWO defect at PCB land 711. No low activityflux barrier was used during reflow soldering in FIG. 7A. FIG. 7B is amicrograph showing a solder joint 720 formed between a BGA and PCB witha NWO defect, including a gap at interface 725 between PCB land 721 andjoint 720. No low activity flux barrier was used during reflow solderingin FIG. 7B. FIG. 7C is a micrograph showing a solder joint 730 formedbetween a BGA and PCB without an NWO defect at PCB land 731. A lowactivity flux barrier, in accordance with implementations, was usedduring reflow soldering in FIG. 7C to suppress NWO defect formation. Asillustrated, connection interface 735 between solder joint 730 and PCBland 730 is similar to that of normal solder joint 710.

FIG. 8A illustrates a BGA solder ball without a flux coating. FIG. 8Billustrates a BGA solder ball after dipping it in a low activity creamyflux, in accordance with implementations. FIG. 8C illustrates a BGAsolder ball after forming a solid coating of a low activity flux overit, in accordance with implementations.

While various embodiments of the disclosed technology have beendescribed above, it should be understood that they have been presentedby way of example only, and not of limitation. Likewise, the variousdiagrams may depict an example architectural or other configuration forthe disclosed technology, which is done to aid in understanding thefeatures and functionality that can be included in the disclosedtechnology. The disclosed technology is not restricted to theillustrated example architectures or configurations, but the desiredfeatures can be implemented using a variety of alternative architecturesand configurations. Indeed, it will be apparent to one of skill in theart how alternative functional, logical or physical partitioning andconfigurations can be implemented to implement the desired features ofthe technology disclosed herein. Also, a multitude of differentconstituent module names other than those depicted herein can be appliedto the various partitions. Additionally, with regard to flow diagrams,operational descriptions and method claims, the order in which the stepsare presented herein shall not mandate that various embodiments beimplemented to perform the recited functionality in the same orderunless the context dictates otherwise.

Although the disclosed technology is described above in terms of variousexemplary embodiments and implementations, it should be understood thatthe various features, aspects and functionality described in one or moreof the individual embodiments are not limited in their applicability tothe particular embodiment with which they are described, but instead canbe applied, alone or in various combinations, to one or more of theother embodiments of the disclosed technology, whether or not suchembodiments are described and whether or not such features are presentedas being a part of a described embodiment. Thus, the breadth and scopeof the technology disclosed herein should not be limited by any of theabove-described exemplary embodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; the terms “a” or“an” should be read as meaning “at least one,” “one or more” or thelike; and adjectives such as “conventional,” “traditional,” “normal,”“standard,” “known” and terms of similar meaning should not be construedas limiting the item described to a given time period or to an itemavailable as of a given time, but instead should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, wherethis document refers to technologies that would be apparent or known toone of ordinary skill in the art, such technologies encompass thoseapparent or known to the skilled artisan now or at any time in thefuture.

The presence of broadening words and phrases such as “one or more,” “atleast,” “but not limited to” or other like phrases in some instancesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent. The use of theterm “module” does not imply that the components or functionalitydescribed or claimed as part of the module are all configured in acommon package. Indeed, any or all of the various components of amodule, whether control logic or other components, can be combined in asingle package or separately maintained and can further be distributedin multiple groupings or packages or across multiple locations.

Additionally, the various embodiments set forth herein are described interms of exemplary block diagrams, flow charts and other illustrations.As will become apparent to one of ordinary skill in the art afterreading this document, the illustrated embodiments and their variousalternatives can be implemented without confinement to the illustratedexamples. For example, block diagrams and their accompanying descriptionshould not be construed as mandating a particular architecture orconfiguration.

What is claimed is:
 1. A method, comprising: dispensing solder paste ona pad of a printed circuit board (PCB); mounting a ball grid array (BGA)on the PCB to form an assembly, the BGA comprising a solder ball that iscoated with a flux, wherein mounting the BGA on the PCB comprisesmounting the solder ball coated with the flux on the dispensed solderpaste; and reflow soldering the assembly at a peak temperature ofgreater than 180° C. to form a solder joint directly coupled to the pad,wherein during reflow, the BGA or PCB thermally warps, causing thesolder ball coated with flux to lift away from the pad, wherein the fluxacts as a barrier that prevents the solder paste from sticking to thesolder ball and lifting away from the pad with the solder ball, whereinafter lifting away, the solder ball collapses back onto the solder pasteand the solder paste reflows into the solder ball to form the solderjoint.
 2. The method of claim 1, further comprising: coating the solderball of the BGA in the flux prior to mounting the BGA on the PCB.
 3. Themethod of claim 2, wherein coating the solder ball of the BGA in theflux comprises: dipping the solder ball in a bath of the flux; sprayingthe flux on the solder ball; or dispensing the flux on the solder ball.4. The method of claim 3, wherein coating the solder ball of the BGA inthe flux further comprises: curing the flux to form a solid coating ofthe flux over the solder ball.
 5. The method of claim 4, wherein coatingthe solder ball of the BGA in the flux comprises: placing the BGA on atray, the tray comprising an opening at a bottom surface exposing thesolder ball; and dipping the exposed solder ball of the BGA placed onthe tray in a liquid or cream of the flux.
 6. The method of claim 4,wherein curing the flux to form the solid coating of the flux comprisesbaking the BGA in an oven at a temperature between 140° C. and 160° C.7. The method of claim 1, wherein the flux is a no-clean flux andwherein the solder paste is a no-clean solder paste.
 8. The method ofclaim 7, wherein the flux consists essentially of: 15 to 85 wt % ofsolvents; 10 to 90 wt % resin; and 0.1 to 10 wt % of a thickening agent.9. The method of claim 8, wherein the flux consists essentially of: 30to 40 wt % of butyl carbitol; 58 to 68 wt % of resin; and 1 to 3 wt % ofa thickening agent.
 10. The method of claim 1, wherein the flux is awater soluble flux, and wherein the solder paste is a water solublesolder paste.
 11. The method of claim 1, wherein during reflow, a fluxof the solder paste dispensed on the PCB penetrates through the fluxcoating the solder ball to remove oxides and promote wetting on thesolder ball to form the solder joint.
 12. A method, comprising:dispensing solder paste on a pad of a printed circuit board (PCB);mounting a ball grid array (BGA) on the PCB to form an assembly, the BGAcomprising a solder ball that is coated with a flux, wherein mountingthe BGA on the PCB comprises mounting the solder ball coated with theflux on the dispensed solder paste; and reflow soldering the assembly toform a solder joint, wherein during reflow, the flux prevents theformation of a non-wet open between the solder joint and the pad,wherein the solder paste reflows into the solder ball to form the solderjoint, wherein the flux is a water soluble flux, wherein the solderpaste is a water soluble solder paste, and wherein the flux consistsessentially of: 15 to 85 wt % of solvents; 10 to 70 wt % oftetrakis(2-hydroxypropyl)ethylenediamine; 0.1 to 30 wt % of modifiedpolyethylene glycol ether; 0.1 to 20% wt % of a water soluble polymer;and 0.1 to 10 wt % of a thickening agent.
 13. A method, comprising:dispensing solder paste on a pad of a printed circuit board (PCB);mounting a ball grid array (BGA) on the PCB to form an assembly, the BGAcomprising a solder ball that is coated with a flux, wherein mountingthe BGA on the PCB comprises mounting the solder ball coated with theflux on the dispensed solder paste; and reflow soldering the assembly toform a solder joint, wherein during reflow, the flux prevents theformation of a non-wet open between the solder joint and the pad,wherein the solder paste reflows into the solder ball to form the solderjoint, wherein the flux is a water soluble flux, wherein the solderpaste is a water soluble solder paste, and wherein the flux consistsessentially of: 20 to 30 wt % of butyl carbitol; 30 to 40 wt % oftetrakis(2-hydroxypropyl)ethylenediamine; 20 to 30 wt % of modifiedpolyethylene glycol ether; 6 to 12 wt % of a water soluble polymer; and3 to 6 wt % of a thickening agent.